This invention relates to a sifting machine having a screen and a plurality of spaced leaf springs, each extending immediately below the screen and transversely thereto for acting upon the screen vertically from below. The leaf springs, as components of a vibrator system, engage directly, although loosely, the screen and are agitated by a drive reciprocating in a direction perpendicular to the plane of the screen.
A sifting machine of the above-outlined type is disclosed, for example, in German Pat. No. 1,189,363 where there is described a flat-screen sifting machine for limestone grinding installations or the like. The leaf springs are longitudinally axially displaceably supported at two oppositely located legs of the screen frame. An end of each leaf spring extends beyond the associated frame leg and is disposed, against the action of a return spring, in the travelling path of a rotating lifting cam. As the cam rotates, the extended ends of the leaf springs are continuously oscillated back and forth against the force of the return spring in a direction perpendicular to the plane of the screen. In this manner the leaf springs beat the screen in their entirety and thus alternatingly bulge the screen and move away therefrom. In this manner there is achieved a continuous, alternating flexing of the screen surface proper. If there are differences in the natural frequencies of the leaf springs and the rpm of the rotary cam, there appear irregularities in the screen motion which lead to whip-like impacts imparted on the screen. This prevents an accumulation of the material in certain zones of the screen surface and simultaneously enhances the passage of the material.
The above-outlined principle of operation has been proven to be very advantageous in sifting machines. It has been found, however, that in the course of an extended operation there occurs damage to the screen fabric caused by the leaf springs, particularly along the clamped longitudinal edges. It has been a further disadvantage that because of the cam drive, the operation of the sifting machine is very noisy. Also, in case of an overload, the resulting downward bulge of the screen fabric was likely to cause a downward bending of the leaf springs so that the ends of the leaf springs to be engaged by the cam have lifted to such an extent that they were no longer disposed in the travelling path of the rotating cams.